Aircraft flying at a relatively low altitude may be subject to risks of colliding with hazard obstacles such as utility wires hung from towers and other high-rise structures. Therefore, it is desirable to provide aircraft with a hazard detection and warning system to improve the aircraft safety.
One way to detect a hazardous object uses "passive" sensing devices to detect radiation emitted from the object. For example, light-sensitive detectors such as CCDs may be used to capture the image of a hazard object or an infrared detector to sense the thermal radiation of an object.
Another detection scheme uses "active" systems which emit certain electromagnetic waves to actively search for hazardous objects by detecting the reflected electromagnetic waves. The time delay between transmitting and receiving a signal may be used to determine the distance of an object and the phase information of the reflected signal(s) may be analyzed to indicate the relative direction and shape of the object.
One implementation of such active detection scheme is the Radar (i.e., radio detection and ranging), in which radio signals are used to probe a radio-reflective object. Another implementation uses laser pulses as the probing signals to detect light-reflective objects by sensing the reflected laser pulses. Since the laser pulses play the similar role as the radio signals in Radar, the latter is also known as light detection and ranging or "Lidar".